Interpretive Summary: Contamination of pathogenic bacteria, e.g., E coli O157:H7 in foods may lead to serious public health concerns. To detect pathogens in foods, proper concentration of the target bacteria must be performed. For this purpose, immunomagnetic beads (IMB) have become a method of choice in many research laboratories. IMB can capture target pathogenic bacteria with specific antibodies on the beads. And simple magnets can easily concentrate captured bacteria. However, for practical applications, factors that determine the efficiency of IMB in capturing the E. coli must be identified and optimized. Thus, we have analyzed the capture using the principles of physics, chemistry and microbiology. The theoretical predictions and experimental results were compared. The results allowed us to conclude that beads with larger size and heavier density are better for the capture. The information is valuable for food safety laboratories to develop practical, automated and bead-based methodologies to detect pathogenic bacteria in foods.

Technical Abstract:
Biotinylated anti E. coli O157 antibodies were linked to Streptavidin coated magnetic beads and polystyrene beads of different sizes and densities. These beads were then used to capture cultured E. coli O157:H7. The complexes formed between fluorescein-conjugated anti E. coli O157 antibodies and urease-conjugated anti fluorescein antibodies further labeled bead-captured bacteria. The extent of the bacterial capture was related to the signal intensity of a light addressable potentiometric sensor that measured the pH changes associated with urease- catalyzed hydrolysis of urea. Under the conditions of applying the same amounts of biotinylated anti E. coli O157 antibody to beads, analyses of the capture kinetics indicated that heavier beads exhibited a better efficiency in capturing the target bacteria than that associated with the lighter beads of the same size. Furthermore, with the same density, larger beads diameters were more effective in capturing the bacteria. Since sedimentation volumes for the heavier and larger beads are greater than those of the lighter and smaller beads, more interactions with the target by the heavier and larger beads are expected. Consequently, the observation is in agreement with a hydrodynamic kinetic model that predicts the bacterial capture is controlled by the sedimentation motion of the beads.